This invention relates generally to powder metallurgy, and more particularly to method and apparatus for making metal powder.
There are major continuing needs in the metals industry for improved, lower cost, high performance alloys of various compositions, including stainless steels, tool steels, maraging steels, super alloys, cobalt and nickel base alloys, titanium alloys, aluminum alloys, copper alloys and others. These needs can be met with new methods and equipment that have been developed to press and consolidate metal powders in a manner that provides improved alloys at lower cost, in controlled shapes.
However, if these new methods are to fully satisfy the needs of industry, they also require improved methods for making metal powders as raw materials to meet the following criteria:
(1) High purity--low impurity content, including oxygen;
(2) Fine particle size--for optimum blending with other powders to homogenous alloy compositions;
(3) Pressable particle forms--suitable for cold pressing to required preform shapes;
(4) Consistent high quality;
(5) Availability-in large quantities;
(6) Availability at low cost--because of high yields from starting material; and minimum processing, labor and energy costs.
Prior apparatus used low temperature milling in air or inert atmospheres to break down scrap metals into powder, with the following problems:
(1) Aerodynamic drag reduces both particle velocity and the efficiency of particle breakdown, especially with high velocity impact milling;
(2) Turbulent gas layers at impacting surfaces prevent or impede penetration of small particles to the impacting surfaces where they can be effectively broken down;
(3) Powder particle surfaces are oxidized during milling unless high purity inert gases are used as a protective atmosphere, which can be costly with some methods;
(4) Standard milling methods generally require extensive milling time or recycling for satisfactory total yields from starting material;
(5) Standard milling methods generally are not satisfactory for producing good yields of fine powders below 20 microns diameter.
(6) The fines from metal powders which are milled in a gas atmosphere can be suspended as a dust in the gas, and increase the problems of aerodynamic drag and cushioning of impact surfaces, causing lower powder breakdown rates as well as environmental and equipment contamination.
Prior apparatus has used high pressure air or gas to propel powders and impact them against hard surfaces at high velocities to break the powders down to finer particle sizes. The disadvantages of such prior art are:
(a) If air is used to drive the powders against an impacting surface, oxidation of freshly fractured surfaces can occur from exposure to the O.sub.2 and H.sub.2 O in air.
(b) If an inert gas such as N.sub.2 is used, a large recovery system is required to separate the gas from the powder after breakdown, and to reclaim the gas for economic re-use
(c) Large gas flows are required to generate necessary particle impact velocities for breakdown, and this creates cushioning and turbulent gas layer effects at impacting surfaces that reduce particle impact velocities to give lower breakdown rates and yields, particularly with finer particles.
(d) Overall costs are fairly high because of the energy inefficiencies in compressing and using gases for this type of milling, and because ofpoor yields and the large equipment installations required.